PSE natural gas customers can purchase carbon-offset "blocks" for $4 each. One block is equivalent to the removal of 400 pounds of carbon dioxide. PSE estimates that an average residential customer using 68 therms of natural gas per month would need to purchase two 400 pound blocks in order to offset 100 percent of his natural gas use.

The program's first customer was Seattle University, which began purchasing offsets for 100 percent of its natural gas use on May 1st.

The Carbon Balance Program for natural gas customers complements PSE's existing Green Power Program for electricity consumers.